Rebels killed several policemen in a pre-dawn attack on the capital of south Sudan's oil-rich Upper Nile state Saturday before southern troops drove them back in heavy fighting, officials said.
"There was an attack on Malakal at 4:00 am (0100 GMT). A small group of militia went into the centre of the town and started shooting," Philip Aguer, spokesman for the Sudan People's Liberation Army, said.
"The SPLA pushed them out, forcing them behind the airport. We do not yet have casualty figures," he added.
"No one knows how the rebels got into the town. But we were woken in the middle of the night by heavy fighting. No one was sleeping," said Susan Oyach, an assistant in the state governor's office.
"The militia killed some police. We don't know how many. But then they fled to the airport," she added.
The army spokesman said the rebel group was commanded by a man called Ulony, whose men fought with the SPLA in Owach, west of Malakal, earlier this week.
More than 70 people were killed in those clashes, Aguer said at the time, accusing Ulony of being in the service of the Khartoum government.
South Sudan, which is due to gain international recognition in July after January's referendum backed independence from the north, has witnessed a wave of deadly clashes with rebel groups in recent weeks.
Violence broke out in Malakal in early February, when loyalists of Gabriel Tang, who commanded a pro-Khartoum militia during the 1983-2005 civil war between the north and south, refused to surrender their heavy weapons.
At least 20 people were killed in the fighting that ensued.
Earlier this week, two days of fighting in neighbouring Jonglei state between the SPLA and renegade southern general George Athor also left more than 20 people dead.
Aguer said on Saturday that the SPLA had dislodged Athor from his bases in northern Jonglei and that the situation there was now calm.
During the devastating civil war Khartoum armed militias among southern ethnic groups opposed to the SPLA, which has repeatedly accused the northern authorities of maintaining the policy in a bid to destabilise the region.
Khartoum has in turn accused the southern authorities of backing rebel groups in the western region of Darfur, something they deny.
Analysts have said maintaining security in the southern nation-in-waiting and disarming its civilian population will be major challenges for the authorities.